The amplitude of the ripple of the output voltage of a DC-DC converter depends on the characteristics of the load supplied by the converter and on the maximum current that may flow through the step-up inductor. FIG. 1 illustrates a typical functioning scheme of a hysteretic step-up converter. With this type of control, the switch N1 is turned on and off with a certain fixed duty-cycle δ as long as FB<FBREF,
  δ  =            T      ON                      T        ON            +              T        OFF            TON being the duration of the charge phase, and TOFF being the duration of the discharge phase of the inductor. Usually, the maximum duration of the charge phase TON, and the minimum duration of the discharge phase TOFF are pre-established.
A feedback voltage FB, representative of the output voltage (in the depicted case it is generated by a voltage divider), is compared with a reference value FBREF. When the feedback voltage increases and the condition FB=FBREF is met, the switch N1 is turned off, and energy stored up to that moment in the inductor is discharged into the load. After the inductor has discharged, the system remains in a stand-by state until FB<FBREF.
The control block CONTROL turns the switch N1 as a function of a first comparison flag FBCOMP and of a second comparison flag OCPCOMP generated by comparing the voltage LX on the inductor (that is proportional to the current that flows therethrough) with a second reference voltage OCPREF.
The current through the inductor is constantly monitored such that during the phase it increases (TON), the current does not overcome a pre-established threshold (proportional to the voltage OCPREF), such as to limit the input current and preventing the inductor from saturating. Should this happen, the conduction phase of the switch N1 is immediately stopped and the inductor discharges completely. Typical waveforms of the inductor current are illustrated in FIG. 2, for example.
Usually, the duty-cycle δ is a design parameter tied only to VIN and VOUT (and not to the load current) according to the formula:
  δ  =            1      -                                    V            IN                                V            OUT                          ⁢                                                  ⁢                                                ⁢        or        ⁢                                  ⁢                  V          OUT                      =                  V        IN                    1        -        δ            The value of δ as well as the maximum current that may flow through the inductor influence the amplitude of the ripple and the efficiency of the converter. The advantage of this type of hysteretic control is its simplicity, since it does not typically require error amplifiers, nor accurate compensations.
As shown in FIG. 3, if the supply voltage VIN is significantly smaller than the output voltage VOUT, the inductor current increases slowly during the charge phase TON and discharges fast during the discharge phase TOFF (this time is also fixed). If, by contrast, the supply voltage VIN is almost equal to the voltage VOUT, the current increases fast during the TON phase and discharges slowly during the TOFF phase. This causes an abrupt increase of the peak current through the inductor upon reaching the maximum current IMAX that may flow through the inductor. With the same output voltage VOUT, the slope of the current through the inductor is proportional to the supply voltage VIN during the charge phase TON, and to VOUT-VIN during the discharge phase TOFF.
The performance of the converter depends on the external load and on the supply. In particular, there may be functioning conditions in which the output voltage ripple and the dissipated power become relatively large.